1. Field of the Invention
The present invention relates to a photomask, a flare measuring mechanism, a flare measuring method, and an exposing method. More specifically, the present invention relates to a photomask used for measuring flare caused by aberration of projection lens in an exposing apparatus used in a process for manufacturing a semiconductor device; and a flare measuring mechanism, a flare measuring method, and an exposing method using such a photomask.
2. Background Art
Generally in the manufacture of a semiconductor device, pattern transfer using a lithographic process is performed in various stages. In this process, light from an exposing light source is radiated onto a photomask including a desired pattern. After the light is converged in a projection lens, the converged light is radiated onto a wafer. Thereby, the resist on the wafer is exposed. If the resist used is a positive-type resist, the exposed region is partially dissolved and removed during developing. Thus, the mask pattern is transferred onto the wafer.
During developing, however, flare may be caused by fine irregularity of the projection lens penetrated by the exposing light, or by scattered light. Flare may deteriorate the contrast of the exposing light that plays an important role in the formation of element patterns and lower the exposing margin in the exposure of the fine patterns, or may bring about deterioration of the shapes of fine patterns.
Therefore, flare is measured for every mask pattern, and compensation for the effects of flare during exposing has been tried using the measured flare.
FIG. 16 is a sectional view for illustrating a conventional photomask used for measuring flare; FIG. 17 is a top view thereof; and FIGS. 18A-18C are schematic top views showing the change of the shape of the transferred resist pattern when exposure is changed in the exposure using the conventional photomask. exposure using the conventional photomask.
In general, for the calculation of the flare rate, the Kirk method (box-in-box method) is used. The measurement of the flare rate by the Kirk method will be described below referring to FIGS. 16 to 18C.
As FIGS. 16 and 17 show, the photomask used for measuring the flare rate by the Kirk method includes, as in ordinary photomasks, a pattern on a substrate that is transparent to exposing light comprising a light-shielding film such as a chromium film. The layout of the photomask 300 is as follows: A square central light-shielding portion 304 is first formed on the center of a transparent substrate 302, and an open portion 306, which is the portion wherein no light-shielding portions are located, is formed to surround the central light-shielding portion 304. On the peripheral portion of the surface of the transparent substrate 302, outside the open portion 306, a peripheral light-shielding portion 308 surrounds the open portion 306.
After the flare rate is measured, the pattern is transferred using the photomask 300. At this time, exposure is changed to transfer the pattern.
For example, FIG. 18A shows an ordinary transferred pattern, and, if exposure is increased from this state, the quantity of light received by the photoresist increases gradually, and, as FIG. 18B shows, the quantity of the removed resist increases. If the exposure is further increased, the transferred pattern 314 formed by imaging the central light-shielding portion 304 on the wafer gradually becomes smaller, and, finally, the transferred pattern 314 disappears as shown in FIG. 18C.
Here, the flare rate measured by the Kirk method is defined as the following equation (1):Flare rate(%)=X/Y×100(%)  (1)where X is the exposure dosage in the case wherein a pattern is transferred as normally designed as shown in FIG. 18A, and Y is the exposure dosage in the case wherein the transferred pattern 314 corresponding to the central light-shielding portion 304 is disappeared as shown in FIG. 18C.
In other words, in the Kirk method, flare is defined as the ratio of (i) the exposure dosage when the photoresist corresponding to the open portion 306 is removed, adequately leaving the photoresist corresponding to the light-shielding regions 304 and 308, to (ii) the exposure when all the photoresist pattern corresponding to the central light-shielding portion 304 is removed. This is defined utilizing the phenomenon that the larger the flare of the projection lens in the exposing apparatus, the easier the resist pattern of the central light-shielding portion 304 on the center of the mask layout is removed.
However, when the Kirk method is used as described above, if the width of the open portion 306 is reduced to some extent, the resist within the central light-shielding portion 304 no longer disappears. Therefore, in recent exposures for forming increasingly miniaturized patterns, the measurement of the flare rate for the pattern with a narrow opening portion using the Kirk method has become difficult.
In addition, it is considered that flare produced in exposing consists generally of long-range flare and local flare. Further, local flare consists of the factor that causes dimensional variation of the exposed pattern on the wafer separated by several micrometers to several tens of micrometers due to the non-uniformity of the refraction index (midrange flare), and wave aberration inherent to the projection lens (shortrange flare; the disagreement of phase lag such as an anastigmatic, coma, or spherical flare including defocusing or distortion, caused by reticle transmission of exposing diffractive light through various films, such as reticles or lenses) is referred to as local flare. In particular, it is difficult to measure local flare using the conventional Kirk method.
Concurrent with the miniaturization of patterns, exposing light or shorter wavelength has been used, and the use of an F2 laser as exposing light is taken into account. When an F2 laser is used, a conventional projection lens consisting of quartz (SiO2) cannot provide sufficient transmittance. Therefore, the use of the projection lens using fluorite (CaF2) can be considered. However, the projection lens consisting of fluorite (CaF2) has large non-uniformity of refraction index due to double refraction, and large roughness of the lens surface. Therefore, if fluorite (CaF2) is used as the material of the projection lens, more flare occurs as compared with the conventional lens consisting of quartz (SiO2). The flare is divided into several components, depending upon its cause, and becomes complicated. Therefore, accurate measurement of flare by the Kirk method has become still more difficult.
However, as the wavelength of the exposing light becomes shorter,the region affected by local flare becomes smaller, but the intensity of local flare is considered to increase. Therefore, it is considered that the effect of the use of shorter wavelength on the line width of the transferred pattern becomes too large to ignore. It is therefore important to correctly know the effect of local flare.